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Looking to run a little extra later on so it should be able to handle at 500.

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No magmount at all is any good for running that kind of power or anywhere close. Even 100W is pushing it. The amount of RF current that needs to flow far outweighs the ability of the capacitive coupling to the car body the magmount uses so you'll have all kinds of common mode problems as the RF uses the coax instead.

Still wouldn't run 200W because of what I mentioned. The more power you run the better the RF ground you need. You can get to a point where your transmitted signal ends up being far less than it should be simply because there's not a good enough ground to work against. For RF current to flow in an antenna and a signal to be radiated from there has to be a source of electrons to draw from and return to. With a poor RF ground you limit that source/return which means less current flows in the antenna so you put out a weaker signal. You can end up in a situation where your 200W amp running into an antenna system with a poor RF ground puts out a weaker signal than someone running 1/4 of the power into an antenna system with a good RF ground.

Still wouldn't run 200W because of what I mentioned. The more power you run the better the RF ground you need. You can get to a point where your transmitted signal ends up being far less than it should be simply because there's not a good enough ground to work against. For RF current to flow in an antenna and a signal to be radiated from there has to be a source of electrons to draw from and return to. With a poor RF ground you limit that source/return which means less current flows in the antenna so you put out a weaker signal. You can end up in a situation where your 200W amp running into an antenna system with a poor RF ground puts out a weaker signal than someone running 1/4 of the power into an antenna system with a good RF ground.

It doesn't matter who makes the magnet mount, they all follow the same laws of physics and all have the same draw backs. Their is no getting around this.

The weakness in the magnet mount is the distance between the magnet and the metal. A certain level of power (or more specifically voltage) will cause arc over, and this arc over will change the impedance of the capacitive coupling that the magnet mount provides while it is happening, and by extension chainging the SWR the radio sees but only at the higher power levels of output when said arc over is happening. Worse, once this happens once it takes even less power for it to happen in the future, the arc over actually weakens the ability of the material to resist said arc overs.

If you insist on using a magnet mount, about the only thing you can do to increase power handeling is use one that has a layer of rubber, or some other padding, between the magnet and the metal of the vehicle. This will raise the power required for said arc over, but at a cost. As the magnet isn't as close to the metal, it simply will not hold as well, and the capacitance provided will be less, making an existing efficiency problem even worse, as well as potentially creating a common mode currents issue, and if you already have common mode currents flowing, the problem will get worse.

Magnet mounts do not handle lots, or really even moderate amounts, of power. It doesn't matter who makes them, it is how they work.

And when it comes to trying to control "heat loss"...I can tell you what to do to lessen "heat loss" in the antenna, its not actually very difficult, all you need to do is use a longer antenna. Longer antennas require less loading, and by extension have less heat loss. Their will always be some, even if you use a full 1/4 wavelength whip, but the loading used to shorten the antenna length is the big offender here. Now the question is, why are you worried about something as minor as heat loss when you have, comparatively speaking, an elephant in the room that is the magnet mount you insist on using? Seriously, the magnet mounts efficiency problems dwarf the heat loss problem, they aren't even in the same league.

I've got quite a collection of magnetic mount antennas. That includes two old Radio Shack ones from the 1980s with 36" vertical radiators; a more recent Radio Shack magnetic mount from the early 2000s that has a 32" vertical radiator; a Wilson 1000; a Stryker SR5K and a Tram 3500. Each works well within a range of reasonable expectations. I have also been able to talk plenty of skip with each one using nothing but legal power.

I know a little "fire in the wire" can help, but, if you're set on a magnetic mount, I'd say put your money into the best antenna you can buy and forget the linear for all of the reasons others have explained.

And for the record, I've got a collection of 102" stainless steel whips, too. Radio Shack "stainless" is definitely not in the same league as U.S. navy "stainless."

There's two hundred different types of stainless steel utilized by the Navy for various applications. ASTM sets the standards and the manufacturers have to comply with the standards. The link you posted is from one of the manufacturers.
There's a huge difference between 304 stainless steel and 316l stainless steel as well as 17 - 7 pH stainless steel. The Navy utilizes all of those for various applications. Yes Metallurgy is one of my many interests.https://www.astm.org/Standards/steel-standards.html

It reminds me of these people that use words like aircraft aluminum which has absolutely zero meaning.

My point wasn't to hijack this thread, but just to make a simple point related to the 102" whip option.

Given our environment here in the Florida Keys stainless steel is an important subject for obvious reasons. My point was simply that the (17-7) stainless steel Radio Shack used on their 102" whips - and I have about 11 of them in various applications- is far lesser able to withstand the elements than the U.S. navy 316L we see lots of around here.

So, in the context of this thread, I agree that a 102" whip is the preferred best solution for what the author ultimately wants to do. However, if he's committed to using a magnetic mount, my opinion is simply that he's best off investing in the best quality one he can get and avoid the linear for all of the reasons folks have already mentioned.

My point was simply that the (17-7) stainless steel Radio Shack used on their 102" whips - and I have about 11 of them in various applications- is far lesser able to withstand the elements than the U.S. navy 316L we see lots of around here.

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Each one of those metals has quite unique properties. 316L is very hard and brittle but very corrosion resistant. Not quite suitable for a flexible whip. 17-7PH is less corrosion resistant, but very flexible and returns back to its original shape after bending. Very different metals with very different purposes.

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